This invention relates generally to automotive emissions testers and particularly to automotive emissions testers that are used with or in computerized engine testing equipment such as that described in the above mentioned related patent applications. The recently introduced Modular Computer Analyzer, known as the Sun MCA 3000, utilizes an IBM compatible computer in conjunction with a data acquisition system (DAS) that incorporates flagged data and an analog to digital (A/D) RAM that is accessible by both the DAS controller and a system microprocessor. A feature of the MCA 3000 is its ability to perform automotive emissions testing in a simpler, faster and more reliable manner.
A so-called emissions test bench uses infrared light and appropriate optical filters to develop an analog voltage that is indicative of the concentrations of particular gases. The particular gases are hydrocarbons (HC), carbon monoxide (CO), and carbon dioxide (CO2). An oxygen (O2) concentration measurement may also be obtained, but the apparatus therefor is not part of the present invention.
Automotive emissions testing, while generating extreme interest recently, is not new. The particular emissions test bench used with the present invention has also been in use for some time and is not part of the invention.
With prior art emissions test bench assemblies, a large number of potentiometers were required for factory and field calibration. It was also advisable to periodically recalibrate testers with a sample gas (cal gas) of known concentration. The cal gas is actually a mixture of Hexane (chemical formula C6H14), CO, and CO2 which provides the different hydrocarbons included in the tests. Recalibration required adjustments of the various potentiometers.
The emissions test bench is essentially a spectral photometric gas analyzer that uses a beam of infrared light to determine concentrations of various gases. The gases absorb different amounts of light at different wavelengths and the amount of light absorbed at a specific wavelength is related to the concentration, i.e. the number of molecules of the specific gas. The bench includes optical filters and sensors. The voltage output of the test bench is non-linear and the number of molecules in a sample cell of gas will change depending upon the pressure in the sample cell.
The prior art IR test bench assembly is equipped with amplifiers to increase its voltage output or span. The normal voltage span for such a bench assembly is from 0 to 10 volts. In the MCA 3000 automotive computer analyzer, the DAS converter has a range of from -10 to +10 volts. In accordance with one aspect of the invention, the emissions test bench assembly output voltage is offset to substantially center it within the range of the A/D converter. According to another aspect of the invention, any gain factor that is present in the bench assembly is measured. Information concerning the calibration pressure, the offset voltage and the gain factor is stored in non-volatile memory and is made accessible to the system microprocessor. These data are thus available to the computer and are used, in accordance with the invention, to self-compensate the test bench assembly output for any difference in pressure between the sample gas and the calibration pressure and to automatically compensate for the zero offset of the test bench assembly and any gain factor each time the bench is recalibrated. By utilization of all aspects of the invention, the emissions test bench assembly rarely, if ever needs to be recalibrated in the field. Also as will be seen, the type of calibration required in the field is simple, automatic for the most part and undemanding on the part of the user.